KR101176296B1 - Aluminum pastes and use thereof in the production of silicon solar cells - Google Patents

Abstract

An aluminum paste comprising particulate aluminum, a tin-organic component, and an organic vehicle, and the use of an aluminum paste in forming a p-type aluminum back electrode of a silicon solar cell.

Description

The use in the manufacture of aluminum pastes and silicon solar cells {ALUMINUM PASTES AND USE THEREOF IN THE PRODUCTION OF SILICON SOLAR CELLS}

The present invention relates to aluminum pastes and their use in the production of silicon solar cells, ie in the production of aluminum back electrodes and individual silicon solar cells of silicon solar cells.

Conventional solar cell structures with a p-type base typically have a cathode on the front or sun side of the cell and an anode on the back. It is well known that radiation of the appropriate wavelength falling at the p-n junction of a semiconductor body acts as an external energy source for generating hole-electron pairs in the body. The potential difference present in the p-n junction moves holes and electrons across the junction in the opposite direction, resulting in a current flow that can transfer power to external circuitry. Most solar cells are in the form of silicon wafers with metallized, ie, electrically conductive, metal contacts.

During the formation of silicon solar cells, aluminum paste is generally screen printed and dried on the backside of the silicon wafer. The wafer is then baked at a temperature above the melting point of aluminum to form an aluminum-silicon melt, followed by the formation of an epitaxially grown layer of silicon doped with aluminum during the cooling step. This layer is commonly called a back surface field (BSF) layer and helps to improve the energy conversion efficiency of solar cells.

Most power-generating solar cells in use today are silicon solar cells. Process flow in mass production is generally aimed at achieving maximum simplification and minimizing manufacturing costs. In particular, the electrode is produced from a metal paste by using a method such as screen printing.

An example of such a manufacturing method is described below in conjunction with FIG. 1. 1A shows a p-type silicon substrate 10.

In FIG. 1B, an n-type diffusion layer 20 of reverse conductivity type is formed by thermal diffusion such as phosphorus (P). Phosphorus oxychloride (POCl ₃ ) is commonly used as the gaseous phosphorus diffusion source, other liquid sources are phosphoric acid and the like. Without any particular modification, a diffusion layer 20 is formed over the entire surface of the silicon substrate 10. a pn junction is formed where the concentration of the p-type dopant is equal to the concentration of the n-type dopant; Conventional cells having pn junctions near the solar side have a junction depth of 0.05 to 0.5 μm.

After formation of this diffusion layer, excess surface glass is removed from the remaining surface by etching with an acid such as hydrofluoric acid.

Next, an antireflective coating (ARC) is applied on the n-type diffusion layer 20 to a thickness of 0.05 to 0.1 μm by a method such as plasma chemical vapor deposition (CVD), for example, in FIG. 1D ( 30) is formed.

As shown in FIG. 1E, a front silver paste (front electrode forming silver paste) 500 for the front electrode is screen printed onto the antireflective coating 30 and then dried. In addition, backside silver or silver / aluminum paste 70 and aluminum paste 60 are then screen printed (or applied in some other application method) to the backside of the substrate and subsequently dried. Typically, the back silver or silver / aluminum paste is first screen printed onto silicon as two parallel strips (busbars) or rectangles (tabs) prepared to solder the interconnect strings, and then the aluminum paste is back silver or silver It is printed on the bare area with a slight overlap on aluminum. In some cases, silver or silver / aluminum paste is printed after the aluminum paste is printed. Subsequently, firing is typically performed in a belt furnace for 1 to 5 minutes, at which time the wafer reaches a peak temperature in the range of 700 to 900 ° C. The front electrode and back electrode may be fired sequentially or cofired.

As a result, as shown in FIG. 1F, molten aluminum from the paste dissolves silicon during the firing process, which then forms an eutectic layer epitaxially growing from the silicon base 10 upon cooling to a high concentration of A p + layer 40 containing aluminum dopant will be formed. This layer is commonly called a back field (BSF) layer and helps to improve the energy conversion efficiency of solar cells. A thin layer of aluminum is generally present on the surface of this epitaxial layer.

The aluminum paste is converted into the aluminum back electrode 61 by firing from the dried state 60. The backside silver or silver / aluminum paste 70 is fired at the same time, resulting in a silver or silver / aluminum backside electrode 71. During firing, the boundary between the backside aluminum and the backside silver or silver / aluminum indicates an alloy state and is likewise electrically connected. The aluminum electrode occupies most of the area of the back electrode, due in part to the need to form the p + layer 40. The silver or silver / aluminum back electrode is formed over a portion of the back side (often as a busbar 2-6 mm wide) as an electrode for interconnecting solar cells by pre-soldered copper ribbons or the like. Additionally, the front silver paste 500 may be sintered during firing and penetrate the antireflective coating 30, thereby allowing electrical contact with the n-type layer 20. This type of process is commonly referred to as "firing through." This plastic conduction state is well illustrated in layer 501 of FIG. 1F.

Silicon solar cells made from silicon wafers and having aluminum back electrodes are silicon / aluminum bimetallic strips and may exhibit so-called bowing behavior. Warpage is undesirable in that it can lead to cracking and breakage of the solar cell. Warping also causes problems with the processing of silicon wafers. During processing, silicon wafers are generally lifted up using automated handling equipment that uses suction pads, which cannot operate reliably in case of excessive warpage. Warping requirements in the photovoltaic industry are typically deflection less than 1.5 mm of solar cells. It is desired to overcome the warpage of large and / or thin-film silicon wafers, for example silicon wafers produced from less than 180 μm thick, in particular from 120 to 180 μm thick, with areas ranging from more than 250 to 400 cm 2. It is a challenge especially when considering silicon solar cells.

A further problem associated with aluminum pastes is the dusting and transfer of free aluminum or alumina dust to other metal surfaces, thereby reducing the solderability and tack of the ribbon tabbed to the surface. This is particularly relevant when the firing process is performed on stacked solar cells.

US Patent Application Publication US-A-2007 / 0079868 discloses an aluminum thick film composition that can be used to form the aluminum back electrode of a silicon solar cell. Apart from the aluminum powder, the organic medium as the vehicle and the glass frit as an optional component, the aluminum thick film composition comprises amorphous silicon dioxide as an essential component. Amorphous silicon dioxide, in particular, serves to reduce the bending behavior of silicon solar cells.

Aluminum thick film compositions having similar or even better performance when the aluminum thick film compositions disclosed in US Patent Application Publication No. US-A-2007 / 0079868 include certain tin-organic components in place of or in addition to amorphous silicon dioxide. It has now been found that this can be obtained.

The present invention relates to an aluminum paste (aluminum thick film composition) for use in forming a p-type aluminum back electrode of a silicon solar cell. The invention further relates to the formation and use of aluminum pastes in the manufacture of silicon solar cells and to silicon solar cells themselves.

The present invention relates to an aluminum paste comprising particulate aluminum, tin-organic components, organic vehicles, and optionally, zinc-organic components, one or more glass frit compositions and amorphous silicon dioxide.

The present invention further relates to a method for forming a silicon solar cell using a silicon wafer having a p-type region and an n-type region, and a pn junction, and the silicon solar cell itself, which method comprises the aluminum paste of the silicon wafer. Applying to the backside, in particular screen-printing, and firing the printed surface so that the wafer reaches a peak temperature in the range of 700 to 900 ° C.

≪ 1 >
1 is a process flow diagram illustratively showing the fabrication of a silicon solar cell.
Reference numerals shown in FIG. 1 are described below.
10: p-type silicon wafer
20: n-type diffusion layer
30: antireflective coating, for example SiNx, TiOx, SiOx
40: p + layer (rear field, BSF)
60: aluminum paste formed on the back side
61: aluminum back electrode (obtained by firing rear aluminum paste)
70: silver or silver / aluminum paste formed on the back side
71: silver or silver / aluminum back electrode (obtained by firing back silver or silver / aluminum paste)
500: silver paste formed on the front surface
501: silver front electrode (obtained by firing the front silver paste)
2a to 2d
2A-2D illustrate a manufacturing process for producing a silicon solar cell using the electrically conductive aluminum paste of the present invention. Reference numerals shown in FIG. 2 are described below.
102 Silicon Substrate (Silicone Wafer)
104 Light receiving surface side electrode
106 Paste Composition for the First Electrode
108 Electroconductive Paste for Second Electrode
110 first electrode
112 second electrode

The aforementioned aluminum dusting problem can be minimized or even eliminated using the novel aluminum thick film composition. The use of the novel aluminum thick film composition in the production of aluminum back electrodes of silicon solar cells not only exhibits low bending behavior and excellent electrical performance, but also reduces or even eliminates the tendency of adhesion loss between the aluminum back electrodes and silicon wafer substrates. Silicon solar cells are produced. The good adhesion between the aluminum back electrode and the silicon wafer substrate leads to an increase in the durability or service life of the silicon solar cell.

The aluminum paste of the present invention comprises particulate aluminum, tin-organic components and organic vehicles (organic medium). In different embodiments, the aluminum face also includes one or more glass frits, zinc-organic components, or one or more glass frits and zinc-organic components.

Particulate aluminum may be composed of aluminum or an aluminum alloy with one or more other metals such as, for example, zinc, tin, silver and magnesium. In the case of aluminum alloys, the aluminum content is, for example, 99.7 to less than 100% by weight. Particulate aluminum can include various shaped aluminum particles, such as aluminum flakes, spherical aluminum powder, nodular-shaped aluminum powder, or any combination thereof. In one embodiment, the particulate aluminum is in the form of aluminum powder. The aluminum powder exhibits an average particle size (average particle diameter) measured by laser scattering, for example of 4 to 10 μm. The particulate aluminum may be present in the aluminum paste of the present invention in a proportion of 50 to 80% by weight, or in one embodiment in a proportion of 70 to 75% by weight based on the total aluminum paste composition.

All references made in this specification and in the claims with respect to the average particle size relate to the average particle size of the relevant materials present in the aluminum paste composition.

Particulate aluminum present in the aluminum paste may involve other particulate metal (s) such as, for example, silver or silver alloy powder. The proportion of such other particulate metal (s) is, for example, from 0 to 10% by weight, based on the total amount of particulate aluminum + particulate metal (s).

The aluminum paste of the present invention comprises a tin-organic component, in one embodiment the tin-organic component may be a liquid tin-organic component. The term "tin-organic component" as used herein refers to solid tin-organic components and liquid tin-organic components. The term "liquid tin-organic component" means a solution of one or more tin-organic compounds in the organic solvent (s), or in one embodiment one or more liquid tin-organic compounds themselves.

In the context of the present invention, the term “tin-organic compound” includes tin compounds comprising at least one organic moiety in the molecule. The tin-organic compounds are stable or essentially stable in the presence of oxygen or air moisture in the atmosphere, for example under conditions prevailing during the preparation, storage and application of the aluminum pastes of the invention. The same is true under the conditions of application of the aluminum paste to the back side of the silicon wafer, especially under conditions prevailing during screen printing of the aluminum paste. However, during firing of the aluminum paste, the organic portion of the tin-organic compound will be removed or essentially removed, eg burned and / or carbonized. The tin-organic compound per se alone has a tin content in the range of 25 to 35% by weight in one embodiment. Tin-organic compounds may comprise covalent tin-organic compounds, in particular they include tin-organic salt compounds. Examples of suitable tin-organic salt compounds include, in particular, tin resinates (tin salts of acid resins, in particular resins having carboxyl groups) and tin carboxylates (tin carboxylic acid salts). In one embodiment, the tin-organic compound may be tin (II) octoate, or more precisely tin (II) 2-ethylhexanoate, which is liquid at room temperature. Tin (II) 2-ethylhexanoate is commercially available from Rohm and Haas, for example. In the case of liquid tin-organic compounds such as tin (II) 2-ethylhexanoate, the undissolved liquid tin-organic compound (s) per se can be used when preparing the aluminum paste of the invention, (II) 2-ethylhexanoate may form a liquid tin-organic component.

The tin-organic component may be present in the aluminum paste of the present invention in proportions corresponding to 0.01 to 0.5% by weight, or in one embodiment 0.1 to 0.15% by weight tin contribution, based on the total aluminum paste composition. In the case of tin (II) 2-ethylhexanoate, the proportion in the aluminum paste may range from 0.1 to 1% by weight or in one embodiment from 0.3 to 0.5% by weight, based on the total aluminum paste composition.

The aluminum paste of the present invention may further comprise a zinc-organic component, and in one embodiment the zinc-organic component may be a liquid zinc-organic component. The term "zinc-organic component" as used herein refers to solid zinc-organic compounds and liquid zinc-organic components. The term "liquid zinc-organic component" means a solution of one or more zinc-organic compounds in the organic solvent (s), or in one embodiment one or more liquid zinc-organic compounds themselves.

In a non-limiting embodiment the zinc-organic component of the aluminum paste of the present invention contains virtually no unoxidized zinc metal, and in further embodiments the zinc-organic component contains more than 90% non-oxidizing zinc metal. In a further embodiment, the zinc-organic component may not contain more than 95%, 97% or 99% non-oxide metal. In one embodiment, the zinc-organic component may not contain non-oxidizing zinc metal.

In the context of the present invention, the term “zinc-organic compound” includes such zinc compounds comprising at least one organic moiety in the molecule. Zinc-organic compounds are stable or essentially stable in the presence of oxygen or air moisture in the atmosphere, for example under conditions prevailing during the manufacture, storage and application of the aluminum pastes of the invention. The same is true under the conditions of application of the aluminum paste to the back side of the silicon wafer, especially under conditions prevailing during screen printing of the aluminum paste. However, during firing of the aluminum paste, the organic portion of the zinc-organic compound will be removed or essentially removed, for example burned and / or carbonized. The zinc-organic compound per se alone has a zinc content in the range of 15 to 30% by weight in one embodiment. Zinc-organic compounds may comprise covalent zinc-organic compounds, in particular they include zinc-organic salt compounds. Examples of suitable zinc-organic salt compounds include, in particular, zinc resinates (zinc salts of acidic resins, in particular resins having carboxyl groups) and zinc carboxylates (zinc carboxylic acid salts). In one embodiment, the zinc-organic compound may be zinc neodecanoate, which is liquid at room temperature. Zinc neodecanoate is commercially available from, for example, Shepherd Chemical Company. In the case of liquid zinc-organic compounds such as zinc neodecanoate, inexpensive liquid zinc-organic compound (s) can be used when preparing the aluminum paste of the invention, and zinc neodecanoate is liquid zinc- Organic components can be formed.

In the case of an aluminum paste comprising a zinc-organic component, the zinc-organic component is aluminum at a ratio corresponding to a zinc contribution of 0.05 to 0.6% by weight or in one embodiment 0.1 to 0.25% by weight, based on the total aluminum paste composition. May be present in the paste. In the case of zinc neodecanoate, its proportion in the aluminum paste may range from 0.5 to 3.0 weight percent, or in one embodiment 0.5 to 1.2 weight percent, based on the total aluminum paste composition.

In one embodiment, the aluminum paste of the present invention comprises at least one glass frit composition as an inorganic binder. The glass frit composition may contain PbO and in one embodiment the glass frit composition may be lead-free. Glass frit compositions may include those which undergo recrystallization or phase separation upon firing and liberate frits having a separated phase having a lower softening point than the original softening point.

The (original) softening point of the glass frit composition (glass transition temperature, measured by differential thermal analysis (DTA) at a heating rate of 10 K / min) may range from 325 to 600 ° C.

The glass frit exhibits an average particle size (average particle diameter) measured by laser scattering, for example of 2 to 20 μm. For aluminum pastes comprising glass-frit (s), the glass frit (s) content is 0.01 to 5% by weight, or in one embodiment 0.1 to 2% by weight, or additionally based on the total aluminum paste composition. In embodiments, 0.2 to 1.25 weight percent.

Some of the glass frits useful for aluminum pastes are conventional in the art. Some examples include borosilicate and aluminosilicate glass. Examples further include combinations of oxides such as B ₂ O ₃ , SiO ₂ , Al ₂ O ₃ , CdO, CaO, BaO, ZnO, Na ₂ O, Li ₂ O, PbO, and ZrO ₂ , which are independently Or may be used in combination to form a glass binder.

Conventional glass frits may be borosilicate frits, for example lead borosilicate frits, bismuth, cadmium, barium, calcium, or other alkaline earth metal borosilicate frits. The preparation of such glass frits is well known and consists, for example, of melting the components of the glass in the form of oxides of the components together and pouring the molten composition into water to form the frit. Of course, the batch components can be any compound that will produce the desired oxide under the usual conditions of frit preparation. For example, boron oxide will be obtained from boric acid, silicon dioxide will be produced from flint, and barium oxide will be produced from barium carbonate.

The glass may be milled in a ball mill containing water or an inert low viscosity, low boiling organic liquid to reduce the particle size of the frit and to obtain a frit of substantially uniform size. It can then be precipitated in water or the organic liquid to separate fines, and the supernatant fluid containing the fines can be removed. Other methods of classification can likewise be used.

Glass is produced by conventional glass making techniques by mixing the desired ingredients in the desired proportions and heating the mixture to form a melt. As is well known in the art, the heating may be carried out for a period of time during which the melt becomes completely liquid and homogeneous and at peak temperature.

The aluminum paste of the present invention may comprise amorphous silicon dioxide. Amorphous silicon dioxide is a finely divided powder. In one embodiment, it may have an average particle size (average particle diameter) measured by laser scattering, for example between 5 and 100 nm. In particular, this includes synthetically produced silica, for example pyrogenic silica or silica produced by precipitation. Such silicas are supplied by various manufacturers in a wide variety of types.

When the aluminum paste of the present invention comprises amorphous silicon dioxide, the amorphous silicon dioxide may, for example, be greater than 0 to 0.5% by weight, for example 0.01 to 0.5% by weight, or one embodiment, based on the total aluminum paste composition. May be present in the aluminum paste in a proportion of 0.05 to 0.1 wt%.

The aluminum paste of the present invention includes an organic vehicle. A wide variety of inert viscous materials can be used as the organic vehicle. The organic vehicle may be one in which the particulate components (particulate aluminum, amorphous silicon dioxide if present, glass frit if present) are dispersible with an adequate degree of stability. The properties of the organic vehicle, in particular the rheological properties, make it suitable for aluminum paste compositions, suitable viscosity and thixotropy for stable dispersion of insoluble solids, applications, in particular screen printing, suitable wettability of silicon wafer substrates and paste solids. ), Such as providing good application properties, including good drying rates, and good firing properties. The organic vehicle used in the aluminum paste of the present invention may be a non-aqueous inert liquid. The organic vehicle may be an organic solvent or an organic solvent mixture, and in one embodiment the organic vehicle may be a solution of organic polymer (s) in organic solvent (s). In one embodiment, the polymer used for this purpose may be ethyl cellulose. Other examples of polymers that can be used alone or in combination include poly (meth) acrylates of ethylhydroxyethyl cellulose, wood rosin, phenol resins and lower alcohols. Examples of suitable organic solvents include ester alcohols and terpenes such as alpha- or beta-terpineol, or kerosene, dibutylphthalate, diethylene glycol butyl ether, diethylene glycol butyl ether acetate, hexylene glycol and high boiling points Mixtures thereof with other solvents such as alcohols. Additionally, volatile organic solvents may be included in the organic vehicle that promote rapid cure after application of aluminum paste to the backside of the silicon wafer. Various combinations of these and other solvents may be formulated to obtain the desired viscosity and volatility requirements.

The organic solvent content in the aluminum paste of the present invention may be in the range of 5-25 wt%, or in one embodiment 10-20 wt%, based on the total aluminum paste composition. Values of 5 to 25% by weight include possible organic solvent contributions from liquid tin-organic components and optional liquid zinc-organic components.

The organic polymer (s) may be present in the organic vehicle in a proportion ranging from 0 to 20 wt%, or in one embodiment 5 to 10 wt%, based on the total aluminum paste composition.

The aluminum paste of the present invention may comprise one or more organic additives, for example surfactants, thickeners, rheology modifiers or stabilizers. The organic additive (s) can be part of an organic vehicle. However, it is also possible to add the organic additive (s) separately when preparing the aluminum paste. The organic additive (s) may be present in the aluminum pastes of the present invention, for example in a total proportion of 0 to 10% by weight, based on the total aluminum paste composition.

The organic vehicle content in the aluminum paste of the present invention may depend on the method of applying the paste and the type of organic vehicle used, which may vary. In one embodiment, it may be 20 to 45 weight percent based on the total aluminum paste composition, or in one embodiment may be in the range of 22 to 35 weight percent. Values of 20 to 45% by weight include organic solvent (s), possible organic polymer (s) and possible organic additive (s).

In one embodiment, the aluminum paste of the present invention

70 to 75 weight percent particulate aluminum,

Tin-organic component (s) in a proportion corresponding to 0.1 to 0.15 wt% tin contribution, in particular 0.3 to 0.5 wt% tin (II) 2-ethylhexanoate,

0.2 to 1.25 weight percent of one or more glass frits,

0-0.5% by weight of amorphous silicon dioxide,

10-20% by weight of one or more organic solvents,

5-10% by weight of one or more organic polymers, and

0-5% by weight of one or more organic additives.

In one embodiment, the aluminum paste of the present invention

70 to 75 weight percent particulate aluminum,

Tin-organic component (s) in a proportion corresponding to 0.1 to 0.15 wt% tin contribution, in particular 0.3 to 0.5 wt% tin (II) 2-ethylhexanoate,

Zinc-organic component (s) in a proportion corresponding to a zinc contribution of 0.1 to 0.25% by weight, in particular 0.5 to 1.2% by weight of zinc neodecanoate,

0.2 to 1.25 weight percent of one or more glass frits,

0-0.5% by weight of amorphous silicon dioxide,

10-20% by weight of one or more organic solvents,

5-10% by weight of one or more organic polymers, and

0-5% by weight of one or more organic additives.

The aluminum paste of the present invention is a viscous composition, which is prepared by mechanically mixing particulate aluminum, tin-organic component, optional zinc-organic component, optional glass frit composition (s) and optional amorphous silicon dioxide with an organic vehicle. Can be. In one embodiment, as a manufacturing method, power mixing, which is a dispersion technique corresponding to traditional roll milling, may be used, and roll milling or other mixing techniques may also be used.

The aluminum pastes of the invention can be used as such or diluted, for example by the addition of additional organic solvent (s); Thus, the weight percentage of all other components of the aluminum paste can be reduced.

The aluminum paste of the present invention can be used for the production of aluminum back electrodes of silicon solar cells or for the production of silicon solar cells, respectively. This preparation can be carried out by applying aluminum paste to the backside of the silicon wafer, ie to a portion of its surface that will or will not be covered, in particular by other backside metal pastes such as backside silver or silver / aluminum paste. Silicon wafers may comprise monocrystalline or polycrystalline silicon. In one embodiment, the silicon wafer may have an area of 100 to 250 cm 2 and a thickness of 180 to 300 μm. However, the aluminum pastes of the invention are larger and / or smaller thickness silicon wafers, for example in the range of less than 180 μm, in particular in the range of 140 to less than 180 μm and / or in the range of more than 250 to 400 cm 2. It can also be used successfully to create aluminum back electrodes on the back side of phosphorus silicon wafers.

The aluminum paste is applied with a dry film thickness of, for example, 15 to 60 μm. The method of applying aluminum paste may be printing, for example silicon pad printing, or screen printing in one embodiment. The applied viscosity of the aluminum paste of the present invention is 20 to 20 when measured at a spindle speed of 10 rpm and 25 ° C. by a utility cup using a Brookfield HBT viscometer and a # 14 spindle. It can be 200 Pa? S.

After application of the aluminum paste to the backside of the silicon wafer, the paste may be dried, for example for a time of 1 to 100 minutes, at which time the wafer will reach a peak temperature in the range of 100 to 300 ° C. Drying can be carried out, for example, using belts, rotary or stationary dryers, in particular IR (infrared) belt dryers.

After application, or in one embodiment, after application and drying, the aluminum paste of the present invention is fired to form an aluminum back electrode. Firing may be performed for a time of, for example, 1 to 5 minutes, in which the silicon wafer will reach a peak temperature in the range of 700 to 900 ° C. Firing can be carried out, for example, using single or multi-zone belt firing furnaces, in particular multi-zone IR belt firing furnaces. Firing takes place in the presence of oxygen, in particular in the presence of air. During firing, organic materials, including organic portions and non-volatile organic materials, which have not evaporated during the possible drying steps can be removed, ie burned and / or carbonized, in particular burned. Organic materials removed during firing include organic solvent (s), possible organic polymer (s), possible organic additive (s), one or more tin-organic compounds and possibly one or more organic parts of zinc-organic compounds. Tin may remain as tin oxide after firing. In one embodiment, the tin oxide after firing may be, for example, SnO, SnO ₂ or a mixture thereof. If the aluminum paste contains a zinc-organic compound, zinc may remain as zinc oxide after firing. If the aluminum paste comprises glass frit (s), there may be an additional process that occurs during firing, ie sintering of the glass frit (s). Firing can be carried out as so-called cofiring with additional metal pastes applied to the silicon wafer, ie applied front and / or back metal pastes, to form front and / or back electrodes on the surface of the wafer during the firing process. One embodiment includes front side silver paste and back side silver or back side silver / aluminum paste.

Next, a non-limiting example of producing a silicon solar cell using the aluminum paste of the present invention is described with reference to FIG. 2.

First, the silicon wafer substrate 102 is prepared. On the light-receiving side (front side surface) of the silicon wafer, typically having a p-n junction near this surface, a front electrode (for example, an electrode mainly composed of silver; 104) is provided (FIG. 2A). On the backside of the silicon wafer, silver or silver / aluminum electroconductive pastes (eg, PV202 or PV502 or PV583 or PV581; available from EI Du Pont de Nemours and Company) ) To form a busbar or tab to allow interconnection with other solar cells set in a parallel electrical configuration. On the backside of the silicon wafer, the novel aluminum pastes of the invention used as backside (or p-type contact) electrodes 106 for solar cells allow for some overlap with the silver or silver / aluminum pastes and the like mentioned above. It is spread out by screen printing using a pattern to be dried and then dried (FIG. 2B). Drying of the paste is carried out, for example, in an IR belt dryer for a time of 1 to 10 minutes, at which time the wafer reaches a peak temperature of 100 to 300 ° C. In addition, the aluminum paste may have a dried film thickness of 15 to 60 μm, and the thickness of the silver or silver / aluminum paste may be 15 to 30 μm. In addition, the overlapped portion of the aluminum paste and the silver or silver / aluminum paste may be about 0.5 to 2.5 mm.

Next, the obtained substrate is fired in a belt firing furnace for a time of, for example, 1 to 5 minutes, at which time the wafer reaches a peak temperature of 700 to 900 ° C., thus obtaining a desired silicon solar cell (FIG. 2d). The electrode 110 is formed from an aluminum paste, where the paste is calcined to remove organic material, and when the aluminum paste comprises glass frit, the glass frit is sintered.

As shown in FIG. 2D, a silicon solar cell obtained by using the aluminum paste of the present invention includes an aluminum electrode 110 and (silver) composed mainly of aluminum on the light-receiving surface (surface) of the silicon base material 102 Or silver or silver / aluminum electrodes 112 composed mainly of silver or silver and aluminum (formed by firing silver / aluminum paste 108) on the back surface.

[Example]

Embodiments mentioned herein relate to thick film metallization paste fired on conventional solar cells having a silicon nitride antireflective coating and a front n-type contact thick film silver conductor.

Although the present invention can be applied to a wide range of semiconductor devices, it is particularly effective for light receiving devices such as photodiodes and solar cells. The discussion below describes how a solar cell is formed using the composition (s) of the present invention and how the solar cell is tested for its technical properties.

(1) manufacture of solar cells

The solar cell was formed as follows:

(i) Si substrate (area 243 cm 2, p-type (boron) bulk silicon with a 20 μm thick silver electrode (PV145 Ag composition available from E. DuPont D. Nemoa & Company) on the front surface; A thick polycrystalline silicon wafer, with Ag / diffused POCl ₃ emitter, acid textured surface, on the backside of SiN _x antireflective coating (ARC) on the emitter of the wafer applied by CVD), Ag / Al paste (PV202, Ag / Al composition available from E. I. DuPont Di Nemoir & Company) was printed and dried on a 5 mm wide bus bar. The aluminum paste for the back electrode of the solar cell was then dried to a dry film thickness of 30 μm, providing overlap of the Ag / Al busbar and aluminum film over 1 mm at both edges to ensure electrical continuity. Screen-printed. The screen-printed aluminum paste was dried and then fired.

The aluminum paste of this example included 72 wt% air-sprayed aluminum powder (average particle size 6 μm), 26 wt% organic vehicle of polymer resin and organic solvent, and 0.07 wt% amorphous silica. The aluminum pastes C to F of this example (according to the invention) contained tin (II) 2-ethylhexanoate additive in the range of 0.1 to 0.5% by weight, while the aluminum pastes A and B of the comparative examples (comparative) No additives of tin-organic compounds were included. Control aluminum paste A did not contain any zinc-organic compounds, whereas control aluminum paste B and aluminum pastes C to F of this example included 1.0% by weight of zinc neodecanoate.

(ii) The printed wafers were then 3000 in a Centrotherm furnace defined with zone temperatures set at 450 ° C. in zone 1, 520 ° C. in zone 2, 570 ° C. in zone 3 and 950 ° C. in the last zone. Firing was performed at a belt speed of mm / min, whereby the wafer reached a peak temperature of 850 ° C. After firing, the metallized wafer became a functional photovoltaic device.

Measurements of electrical performance, fired adhesion and warpage were made.

(2) test procedure

efficiency

For the purpose of measuring the light conversion efficiency, a solar cell formed according to the method described above was placed in a commercial I-V tester (supplied by EETS Ltd.). The lamp in the I-V tester simulated sunlight of known intensity (about 1000 W / m 2) and illuminated the emitters of the cell. The metallization printed on the fired cell was then contacted with four electrical probes. The photocurrent generated by the solar cell (Voc: open circuit voltage, Isc: short circuit current) was measured across various resistors to calculate the I-V response curve. The fill factor (FF) and efficiency (Eff) were then derived from the I-V response curve.

Plastic adhesion

In order to measure the cohesive strength of the Al metallization, the amount of material separated from the surface of the fired wafer was measured using the peel test. For this purpose, a transparent layer of adhesive tape was applied and then peeled off. The adhesion values in Table 1 show the increase in the adhesion of the paste corresponding to the increase in the tin-organic content of the composition. The peel strength of the paste of the example could be further improved by the addition of glass frit.

Warpage measurement

Warpage (cell warpage) is defined as the maximum deflection height of the fired cell center at room temperature when measured on a flat surface. A warpage measurement is performed by placing the cells on a metal flat bed and using a dial gauge with a resolution in μm to measure the maximum deflection of each cell, ie by measuring the distance from the flat surface to the center of the wafer. It was.

Examples C to F mentioned in Table 1 show the electrical properties of aluminum pastes as a function of tin-organic content as compared to compositions that do not contain tin-organic additives (Control A and Control B). The data in Table 1 confirms that warpage is reduced with further improvement of adhesion to Examples C-F.

[Table 1]

Claims (16)

An aluminum paste comprising an organic vehicle comprising particulate aluminum, tin-organic component and organic solvent (s). The aluminum paste of claim 1, further comprising at least one glass frit in a total proportion of 0.01 to 5% by weight based on the total aluminum paste composition. The aluminum paste of claim 1 or 2, further comprising a zinc-organic component in a proportion corresponding to a zinc contribution of 0.05 to 0.6% by weight based on the total aluminum paste composition. The aluminum paste of claim 1 or 2, further comprising amorphous silicon dioxide in a ratio of greater than 0 to 0.5% by weight based on the total aluminum paste composition. The aluminum paste of claim 1 or 2, wherein the particulate aluminum is present in a proportion of 50 to 80% by weight based on the total aluminum paste composition. The tin-organic component of claim 1, wherein the tin-organic component comprises one solid tin-organic compound, a combination of two or more solid tin-organic compounds, one liquid tin-organic compound, a combination of two or more liquid tin-organic compounds , A combination of solid and liquid tin-organic compounds, and a solution of a tin-organic compound of at least one of the organic solvent (s). The aluminum paste of claim 6, wherein the tin-organic component is present in a proportion corresponding to 0.01 to 0.5% by weight tin contribution based on the total aluminum paste composition. 7. The aluminum paste of claim 6, wherein the tin-organic compound (s) is a tin-organic salt compound selected from the group consisting of tin resinates and tin carboxylates. 7. The aluminum paste of claim 6, wherein the tin-organic component is tin (II) 2-ethylhexanoate present in a proportion of 0.1 to 1% by weight based on the total aluminum paste composition. The aluminum paste of claim 1, wherein the organic vehicle further comprises organic polymer (s), organic additive (s), or both. (i) applying the aluminum paste of claim 1 or 2 to the backside of a silicon wafer having a p-type region, an n-type region and a pn junction, and
(ii) firing the surface with the aluminum paste to cause the wafer to reach a peak temperature of 700 to 900 ° C. The method of claim 11, wherein the application of the aluminum paste is performed by printing. 12. The method of claim 11, wherein firing is performed as cofiring with other front metal pastes, back metal pastes, or both applied to the silicon wafer, to form a front electrode, a back electrode, or both on the silicon wafer during firing. How to form. A silicon solar cell produced by the method of claim 11. A silicon solar cell comprising an aluminum back electrode produced using the aluminum paste of claim 1. The silicon solar cell of claim 15, further comprising a silicon wafer.

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